Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the frequency difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
For pulsed radar level gauge systems, time expansion techniques are generally used to resolve the time-of-flight.
Such pulsed radar level gauge systems may have a first oscillator for generating a transmission signal formed by pulses for transmission towards the surface of the product contained in the tank with a transmitted pulse repetition frequency ft, and a second oscillator for generating a reference signal formed by reference pulses with a reference pulse repetition frequency fref that differs from the transmitted pulse repetition frequency by a known frequency difference Δf. This frequency difference Δf is typically in the range of Hz or tens of Hz.
At the beginning of a measurement sweep, the transmission signal and the reference signal are synchronized to have the same phase. Due to the frequency difference Δf, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep.
During the measurement sweep, the reflection signal formed by the reflection of the transmission signal at the surface of the product contained in the tank is correlated with the reference signal, so that an output signal is only produced when a reflected pulse and a reference pulse occur at the same time. The time from the start of the measurement sweep to the occurrence of the output signal resulting from the correlation of the reflection signal and the reference signal is a measure of the phase difference between the transmission signal and the reflection signal, which is in turn a time expanded measure of the time-of-flight of the reflected pulses, from which the distance to the surface of the product contained in the tank can be determined.
Since the accuracy of the frequency difference Δf between the transmission signal and the reference signal is important to the performance of the pulsed radar level gauge system, the second (and/or the first) oscillator can be controlled by a regulator that monitors the frequency difference Δf and controls at least one of the first and the second oscillator to achieve a frequency difference that is known and sufficiently close to the desired frequency difference Δfdes for which the pulsed radar level gauge system is designed.
To provide a stable regulation, the regulator may need in the order of hundreds of samples of the frequency difference Δf which corresponds to a time duration which can be as long as 20-30 seconds due to the low value of the frequency difference Δf that is desired to achieve a sufficient time expansion.
Accordingly, currently available pulsed radar level gauge systems may need to be powered for a substantial period of time before the actual filling level measurement can start.
U.S. Pat. No. 7,412,337 discloses a method aimed at reducing the time needed to control the first and/or second oscillator to achieve the desired frequency difference Δf. In the method according to U.S. Pat. No. 7,412,337, the gradient of at least two control variable-difference frequency data points is determined, and on the basis of the gradient and the difference frequency, an operating point of the control is determined and the control algorithm is adjusted. The method according to U.S. Pat. No. 7,412,337, however, appears relatively complicated and cumbersome, and there also appears to be room for further improvement in respect of reducing the time needed for the control.